Electrolytic cells that are commonly employed commercially for the conversion of alkali metal chloride into alkali metal hydroxide and chlorine may be considered to fall into the three following general types: diaphragm, mercury, and membrane cells. The present invention relates to membrane cells.
Membrane cells utilize one or more membranes or barriers separating the catholyte and the anolyte compartments. The membranes are permselective, that is, they are selectively permeable to either anions or cations. Generally, the permselective membranes utilized are cationically permselective. The catholyte product of the membrane cells is a relatively high purity alkali metal hydroxide. The catholyte product, or cell liquor, from a membrane cell is purer and of a higher concentration than the product of a diaphragm cell.
Membrane-type chlor-alkali cells are sensitive to the presence of sulfate in the feed brine. Alkali metal sulfates migrate from the anolyte compartment through the membrane towards the catholyte compartment. At some concentration, the solubility of the sulfates will be exceeded in the highly alkaline environment of the catholyte compartment. When the sulfates reach a zone of sufficient alkalinity, they precipitate in the membrane, disrupting its structure. Concentrations higher than a few grams per liter in the anolyte compartment will cause deposition of sulfates in the membrane. The degradation of the membrane causes a gradual drop in the current efficiency of the membrane cell and results in the physical failure of the membrane.
This is a problem of fairly recent origin. Diaphragm and mercury cells, which have been the industry standard, are less sensitive to sulfates. The earliest commercial membrane cells were inherently less efficient than those based on today's membranes and so were less susceptible to more subtle effects.
Sulfate in the saturated brine solution originates primarily from calcium sulfate, either anhydrite or gypsum, which occurs naturally in rock salt deposits formed by the evaporation of inland seas and in solar salts. Sulfates may also be present in processed salts including evaporator, recrystallized, and purified vacuum salts. The rock salt may be mined and converted to brine solution in above-ground dissolvers or saturators. Alternatively, it may be dissolved underground by the injection of water or unsaturated brine. In either case, the resulting brine will contain calcium sulfate in quantities ranging upward to saturation. Additionally, sulfates can result from compounds other than calcium sulfate.
There are very few commercial examples of applied solutions to address the problem of controlling sulfate concentration. Two of the methods, purging and precipitation, attempt to remove the sulfate from the saturated brine.
Processes utilizing purge streams, without other treatment, may be costly. Due to the low selectivity of the purge process, the cost of the accompanying sodium chloride in the purge stream is high, and the total dissolved solids in the plant effluent may be intolerable.
Sulfates can be precipitated from the saturated brine by the addition of various compounds for example calcium chloride, barium chloride, or barium carbonate. Increasing the calcium concentration above that which resulted from initial dissolution of the calcium sulfate (CaSO.sub.4) forces some of the sulfate out of solution as calcium sulfate. In the case of the addition of a barium compound, barium sulfate (BaSO.sub.4), which is less soluble than calcium sulfate, is precipitated after treatment with calcium or barium. Sodium carbonate (Na.sub.2 CO.sub.3) can be added, which in turn precipitates the excess calcium or barium as its respective carbonate. The carbonates are filtered off separately from or along with the sulfate. Finally, the brine is subject to an ion exchange treatment to remove last traces of added alkaline earth metal.
The use of barium is very effective in sulfate removal and allows the sulfate concentration to be reduced to very low values. However, this method has serious drawbacks. The cost of barium is high. It is recognized as a toxic substance and is slowly leachable from the precipitate, thereby necessitating special disposal measures. Additionally, the introduction of excess barium (or accompanying strontium) to a brine stream places an additional load on the brine softening process.
Precipitation of sulfate with calcium eliminates the toxicity problem but presents some of its own. CaSO.sub.4 is much more soluble in water than is BaSO.sub.4 (about 2 grams per liter versus 2 milligrams per liter). It is also salted in by sodium chloride (NaCl), being about three times as soluble in saturated brine as in water. Precipitation by calcium is, therefore, less effective than precipitation by barium.
The precipitation treatment procedures are also unsatisfactorily slow. The rate of crystallization of calcium sulfate or barium sulfate is low because of the low concentrations of the two reacting ions, and a relatively long time for precipitation must be allowed. The resulting precipitate may be extremely finely divided and difficult to settle or filter.
Precipitation requires more equipment and processing steps than the present invention. The sulfate must be precipitated and removed separately from the calcium to avoid redissolution of the sulfate. In a typical brine plant, this would mean the addition of a treatment tank. Other equipment would be needed for removal of the solids. Conventional practice is to use an open clarifier, which usually is the largest piece of equipment in the plant. The use of two clarifiers to remove different solids adds significantly to the area occupied by a plant.
Two other known commercial methods of controlling sulfate concentration are the use of "rapid" dissolvers and the use of additives in conventional dissolvers. Both, like the present invention, are intended to restrict the introduction of sulfates to the saturated brine.
The so-called rapid dissolvers take advantage of the relative kinetics of dissolution of NaCl and CaSO.sub.4. The former dissolves much more rapidly than does the latter. If the salt is allowed only a limited time of contact with depleted brine, NaCl will dissolve selectively. The resultant solution will be essentially saturated with NaCl but far from saturated with CaSO.sub.4. The disadvantages of this technique are its sensitivity to processing changes and to the physical form of the salt. Since salt residue must be removed continually, it is necessary to keep the flowrates of salt and depleted brine in close balance. If the operating rate of a plant is reduced, the flowrates of salt and of circulating depleted brine will also decrease. Unless compensating changes are made, the fraction of the CaSO.sub.4 which dissolves will increase due to increased contact time between the salt and the depleted brine. This problem with varying process flowrates does not exist with conventional dissolvers or with the present invention. Rapid dissolution is not feasible if the salt is being dissolved in an underground mine.
Finally, the effectiveness of rapid dissolvers depends on the physical form of the salt. Natural salts will be either rock salts or solar salts. The sulfate content of the former tends to be present as discrete particles of anhydrous CaSO.sub.4 (anhydrite) which are uniformly distributed throughout the salt. Selective dissolving is possible in this case. With solar salts, the CaSO.sub.4 is more evenly distributed (or even on the surface of the particles) and is likely to be present as gypsum (CaSO.sub.4.2H.sub.2 O), which is much more rapidly soluble. Rapid dissolvers tend to be less selective with solar salts because gypsum dissolves more rapidly than anhydrite.
Certain additives, used with conventional dissolvers, inhibit the solubility of CaSO.sub.4. The additives commonly used to prevent dissolution of sulfates are phosphate compounds, detergent types, or a combination of the two. These depend on the fact that sulfates are present as CaSO.sub.4 (MgSO.sub.4) and are in their action suppressors of calcium (magnesium) solubility. It is practical to use them only at very low concentrations. They are, therefore, most effective with rock salts, where a small amount is sufficient to coat a particle of anhydrite with insoluble calcium compounds. By forming an insoluble coating on the surface of the discrete anhydrous CaSO.sub.4 particle, the CaSO.sub.4 is prevented from reaching equilibrium with the brine. When CaSO.sub.4 is widely dispersed or is present as gypsum, these agents are found to be much less effective. Furthermore, their additions may be incompatible with the membrane and adversely affect membrane cell operation.